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Abstract
The stability of Cr/C multilayer during irradiation or thermal annealing was investigated using grazing incidence X-ray reflectivity measurement, X-ray photoelectron spectroscopy, X-ray diffraction analysis, small-angle X-ray scattering analysis, and soft X-ray reflectivity measurement. One sample was irradiated with a white beam of synchrotron radiation and five other samples were annealed at various temperatures. The 18-h irradiation treatment caused local surface contaminants but did not affect the buried stacks. The annealing treatment resulted in increased reflectivity at approximately 1.2 keV, and the multilayer remained stable for temperature up to 700 °C. Thus, the Cr/C multilayers exhibited excellent stability during irradiation and thermal treatments and can be used for the mirrors and multilayer gratings of third-generation synchrotron radiation systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Third-generation synchrotron light source can emit X-ray beams with unprecedentedly high brilliance and coherence and thus has the potential to drive developments in spectroscopy and imaging technology [1–3]. With respect to the “tender X-ray range” (1-4 keV), which corresponds to the K-absorption edges of P, S, Cl and 3d transition metals as well as L-absorption edges of 4d transition metals [4–6], one of the major challenges is the fabrication of monochromator with high efficiency. The traditional monochromators including crystal and single-layer grating have different problems in this range. Crystal monochromator is limited by the small d-spacing, which requires a near-normal incidence angle. However, the resulting heat load can lead lattice distortion. Conventional single-layer coating grating monochromator can disperse “tender X-ray” with an efficiency of only a few percent. Thus, the performance is far inferior to that required [7,8]. Multilayer blazed grating fabricated by depositing high-reflectivity multilayer on blazed grating have been proposed for fabricating monochromator with high efficiency [9–11]. In tender X-ray range, Cr/C multilayer has been reported to be the best coating for mirrors, as it exhibited reflectivity as high as of 60% at 3 keV [12]. Thus, in a previous study, we designed a Cr/C-multilayer-based collimated plane-grating monochromator (cPGM). In principle, the designed cPGM (the period of the multilayer was 11.64 nm) can acquire an efficiency an order of magnitude higher than that of the currently used monochromator, which is coated with single layer gold [13]. However, monochromator is usually placed close to light source, and the power of in-coming beam can be as high as several tens of watts within a small area. Further, in the absence of a cooling system, the temperature of the element can reach hundreds of degrees. Moreover, even when the temperature is controlled to tens of degrees with active cooling, long time exposure can result in significant damages [14]. Therefore, the stability of Cr/C multilayer under irradiation with synchrotron radiation or a heat load must be elucidated before the multilayer can be used as the coating in high efficiency cPGM. Hence, in this study, we attempted to determine the irradiation and thermal stability of Cr/C multilayer.
The irradiation-induced damages caused to mirrors by synchrotron radiation are mainly attributable to the thermal effect and adsorbed surface contaminants [15,16]. The thermal effect is the primary cause of the multilayer structural degradation. The heating of the multilayer to a high temperature by the high-intensity synchrotron radiation can activate the atoms near the interfaces, leading to diffusion-related intermixing of the layers [17]. Moreover, the high temperature may transform the amorphous layer into crystal or cause the recrystallization of polycrystal [18]. Other structural and phase transformations, such as changes in layer thickness and agglomerations as well as chemical reactions, may also take place [19]. Surface adsorption can result local carbonaceous contaminants, which can degrade the efficiency of the optical elements. The growth rate of contaminant is dependent on the exposure time, base pressure, substrate temperature and surface electric field intensity [20]. When X-ray standing wave has an antinode at the multilayer surface, the electric field intensity can be highly enhanced, leading to stronger contaminant adsorption effect. The mechanism of contaminant generation can be divided into three steps: physical adsorption, hydrocarbon dissociation, and the generation of carbonaceous contaminants through a photochemical reaction [21,22]. Several methods for removing these contaminants have been proposed, such as glow discharge treatment in an oxygen atmosphere, plasma discharge treatment in a mixture of oxygen and argon, ultraviolet/ozone cleaning, and cleaning with atomic hydrogen generated using a heated catalyzer [23–26]. However, the stability of Cr/C multilayer under irradiation with synchrotron radiation has never been studied.
Previous investigations on the effects of high temperature on metal/carbon (W/C, Ni/C, Cr/C, Co/C, and Pt/C) multilayers showed that the C layers turned from amorphous to graphitic, resulting in increases in the thicknesses and decreases in the densities [27–29]. In addition, increases in the reflectivities were observed in the case of W/C, Ni/C, and Co/C multilayers after annealing at low temperature (< 500 °C). This possibly was attributed to the smoother interfaces and the enhanced optical contrast between layers. When Cr/C multilayer was annealed at 600 °C for 2 h, a greater amount of Cr-C mixtures was produced at the interfaces, resulting in increases in the interlayer thicknesses [30,31]. However, to our knowledge, the effect of thermal annealing on the Cr layer in Cr/C multilayer remains unclear.
In this study, we evaluated irradiation stability of a Cr/C multilayer consisting of 20 bilayers having a period of approximately 11.64 nm. As a preliminary experiment, the sample was irradiated by a synchrotron radiation white beam with normal power density of 0.1 W/mm2. The irradiation was performed at the National Synchrotron Light Source (NSRL), Hefei, China, for roughly 18 h. We also evaluated the effects of thermal treatment on Cr/C multilayers with same structural parameters as above. Five samples were annealed at 200, 300, 400, 500, and 700 °C for 1 h, respectively, in a vacuum. Finally, the effects of the damages induced by the irradiation and thermal annealing treatments on optical properties were also investigated by comparing the measured reflectivities in the tender X-ray range before and after the stability tests.
2. Experimental
Seven Cr/C multilayers having same structural parameters were deposited on polished, 0.6-mm-thick Si (100) wafers by direct current magnetron sputtering. The vacuum system was evacuated to a background pressure of 5.0×10−5 Pa prior to deposition, while the Ar pressure was fixed at 0.146 Pa during the deposition process. High-purity Cr (99.95%) and C (99.999%) targets were used for the sputtering process. First, the Cr layer (4.0 nm) was deposited on the substrate. Then, 20 C/Cr multilayers were grown on the wafers. The period of the multilayers was approximately 11.64 nm, and the Cr thickness ratio (the ratio of Cr layer thickness to period) was approximately 0.37. Finally, C layer with thickness of 5.5 nm was deposited on the top to prevent the oxidation of the top Cr layer [32].
The irradiation treatment was performed using the white beam from the bending magnet of beamline 08B at the NSRL. Figure 1 shows the schematic of beamline 08B. Two Au-coated toroidal mirrors (Rhorizontal: 55.00 m, Rvertical: 15.41 m) and a spherical grating monochromator (SGM) are used in this beamline. The SGM was set at zero-order position. The first toroidal mirror, which was placed 7.3 m from the source, accepted 5.0 mrad of the horizontal divergence and 1.5 mrad of the vertical divergence of the synchrotron radiation (SR) and deflected the in-coming beam at a grazing angle of 4.0°. The apertures of S1 and S2 remained constant at the maximum values of 600 and 400 µm, respectively, during the irradiation process. The total power of the SR emitted from the bending magnet was approximately 3.48 W for a ring current of 300 mA and electron energy of 0.8 GeV. After the beam passed through the series of mirrors, the power decayed to approximately 0.42 W at position D, where the Cr/C multilayer was placed for the irradiation treatment. The beam size at position D was 1.3 mm×3.2 mm. Therefore, the beam power density was 0.1 W/mm2 when the beam was normally incident on the sample. A vacuum chamber was used to keep the background pressure at 4.0×10−4 Pa. The irradiation treatment was performed under normal incidence and lasted for 18 h.
The grazing incidence X-ray reflectivity (GIXRR) was measured at Cu-Kα line (photon energy E = 8.04 keV) in order to characterize the multilayer structure before the sample was subjected to the irradiation treatment. A constant-period model consisting of Cr and C layers was used to fit the measured curve. This fitting can give the actual thickness ratio of the multilayer. However, the simulated curve did not match well with the measured one because the constant-period model is too simple, which has been described in our previous studies [32,33]. To solve this problem, the measured curve was refitted using a four-layer and non-periodic model that takes into account the Cr-C mixtures formed at the interfaces as well as the slight shifts in the thicknesses of the individual Cr and C layers [32]. The slight shifts were set linear with period number during the fitting process.
We would like to mention that, in this paper, all the reflectivity curve fittings were performed using the software Bede Refs. and IMD [34,35].
The three-dimensional (3D) profile of the irradiated region was obtained by using a Bruker ContourGT optical profiler in the PSI mode at the wavelength of 533.4 nm. Six subapertures measured using a 2.5× objective were stitched into a 3.0 mm × 6.0 mm area.
Surface X-ray photoelectron spectroscopy (XPS) measurements were performed within and outside the irradiated region using an AXIS ULTRADLD system with Al-Kα line as the source at 1464.6 eV. The measurements were performed over an area of 0.3 mm × 0.7 mm. The base pressure during the measurements was less than 7.0×10−7 Pa, and the power applied at the anode was 150 W. The element compositions and chemical states were determined by fitting the photoelectron spectra using the software Fityk 0.9.4 [36].
The GIXRR curve was measured again after irradiation, and the center of the beam was aligned such that it was incident at the center of the irradiated region. The measured curves were compared with the ones obtained before irradiation to elucidate the effects of the irradiation process on the multilayer structure. In addition, the sixth-order peaks of different regions were compared in order to analyze the structure differences between the irradiated region and the area outside it. The related details are given in the section that describes the irradiation test.
The reflectivity was measured along the longer side of the irradiated region in intervals of 0.5 mm at 1183.6 eV, in order to elucidate the irradiation effect on the optical properties. The reflectivity measurements were performed at the BEAR beamline of the Elettra Sincrotrone, Trieste, using s-polarization (polarization level close to 100%). A solid-state photodiode was used to detect the intensities of the reflected photon. The reflectivity was determined by normalizing the intensities of the reflected photons with respect to those of the incident intensities.
To assess the thermal stability of Cr/C multilayer, five samples (marked S1-S5) were annealed at 200, 300, 400, 500, 700 °C, respectively, for 1 h. Sample S6, which was not subjected to the annealing treatment, was used as the reference sample for the X-ray diffraction (XRD) and soft X-ray reflectivity measurement. A vacuum chamber in which the samples could be heated up to 1000 °C was used to keep the background pressure under 2.0×10−4 Pa during the annealing treatment. The temperature was increased at a rate of 10 °C /min. The final temperature was maintained for 1 h, after which the heater was turned off. Subsequently, the samples were allowed to cool down to room temperature naturally in the vacuum chamber.
We measured the GIXRR of all the samples before and after annealing treatment. The structural changes were analyzed by fitting the GIXRR curves with the constant-period model and the four-layer and non-periodic model.
In addition, small angle X-ray scattering (SAXS) measurements were also performed both before and after the annealing treatments, in order to characterize the changes in the interface roughness. The detector scan mode and a fixed grazing incidence angle corresponding to the first-order Bragg peak were used for the SAXS measurements. In addition, out-of-plane XRD measurements were performed in symmetrical reflection mode to determine the changes in degree of crystallization of the samples. The details of the XRD measurements have been reported in our previous study [32].
Finally, reflectivity measurements were performed at grazing angles of 3.0° and 3.6° on the as-deposited (S6) and annealed samples (S1-S5) at the BEAR beamline of the Elettra synchrotron radiation facility.
3. Results and discussion
3.1. Modeling of non-irradiated sample
The GIXRR curve was fitted to characterize the multilayer structure of the sample used for the irradiation test. The Cr thickness ratio as determined by fitting with the constant-period model was 0.37. The other structural parameters, including the densities, interlayer thicknesses, and layer thicknesses, were obtained by fitting the measured curve using the four-layer and non-periodic model. The results are listed in Table 1. The average thicknesses of the bottom and top layers were used to represent as layer thickness since the thickness drifts from period to period were small (lower than 0.06 nm from top to bottom layer) [32]. The measured and fitted curves are shown in Fig. 2. The period was 11.51 nm, which was calculated by summing the thicknesses of the Cr and C as well as both of the various interlayers (Cr-on-C and C-on-Cr). The thicknesses of the Cr-on-C and C-on-Cr interlayers were 1.30 and 0.80 nm, respectively. Based on the fitting results, the density of the C layer was 2.10 g/cm3, which is similar to that reported for sputtered carbon films in other studies [37–39]. The density of the Cr layer, which was 6.67 g/cm3, was slightly lower than that of the Cr bulk (7.19 g/cm3). Finally, the average interface roughness was 0.34 nm.
Fig. 2. Measured (solid black lines) and fitted (red spots) GIXRR curve for Cr/C multilayer sample before irradiation treatment. Fitting was performed using the four-layer and non-periodic model.
Table 1. Structural parameters obtained from fitting of GIXRR curve of Cr/C multilayer sample before irradiation treatment. Thicknesses of individual Cr and C layers are average thicknesses of the top and bottom layers, respectively.
3.2. Irradiation test
An image of the Cr/C multilayer sample after the irradiation treatment is shown in Fig. 3. Contaminants can be seen clearly in the region where the synchrotron radiation beam was incident. There is a light-brown halo around the edge of this region. Figure 4 shows the 3D profile of the surface. The poor vacuum resulted in a spot with contaminant. The height at this spot was approximately 25 nm. The average growth rate of the contaminants was 1.39 nm/h, which is slightly higher than that reported in previous studies [15,16,22,40,41]. The average width and length of the spot were approximately 1.25 and 3.19 mm, respectively, and similar to the beam footprint during the irradiation process. Figure 5 shows the XPS of C, Cr, and Ar spectra as measured within and outside of the irradiated region (the measurement regions are marked as A and B, respectively, in Fig. 4). Characteristic Cr-2p, C-1s, and Ar-2p peaks were observed in the case of the area outside of the irradiated region. The observed Cr-2p characteristic peak corresponded to the Cr layer adjacent to the top C layer. The Ar-2p peak was ascribed to the Ar atoms present as impurities, as Ar was used as background sputtering gas. In case of the irradiated region, the characteristic Ar and Cr peaks disappeared and only the C-1s peak was observed. This suggested that a new C layer (carbonaceous contaminants) had formed locally on the surface. The chemical states of the constituent elements were determined by fitting the characteristic peaks. The results are displayed in Fig. 5. The Cr 2p spectrum contained large peaks at binding energies of approximately 574.5 (Cr 2p3/2) and 583.6 eV (Cr 2p1/2). The peak at 574.5 eV can be attributed to metallic Cr [42], while that at 575.5 eV is assignable to the Cr-C bond [43,44]. Finally, the third peak, which had binding energy of 577.2 eV was attributable to the Cr-O bond [45,46]. The existence of the Cr-C bond confirmed the formation of Cr-C mixtures at the interfaces. The C-1s spectrum measured in region B could be fitted using four components. The two primary peaks, which were located at binding energies of 284.6 and 285.2 eV, corresponded to sp2-C and sp3-C, respectively [47–49]. The C-1s component at 286.4 eV corresponded to the C-O bonds formed because of the adsorbed hydrocarbons [42,50]. Finally, the Cr-C mixtures were responsible for the Cr-C component seen at 283.0 eV [47]. The C-1s spectrum measured in region A did not contain a Cr-C component because no Cr-related signal was observed within the irradiated region. The sp2/sp3 peak area ratio was 2.40 and larger by 0.82 than that in case of the as-deposited top C layer (region B), implying that the generated contaminants contained a greater amount of graphitic carbon.
Fig. 5. Measured and fitted C 1s, Cr 2p and Ar 2p XPS spectra as measured within (Region A) and outside (Region B) irradiated region: (a), (b), (c) within irradiated region and; (d), (e), (f), outside irradiated region. Dots and solid lines represent measured and fitted data, respectively.
The effects of the irradiation treatment on the multilayer structure were also investigated through GIXRR measurements. Figure 6(a) shows the GIXRR curves measured before and after the irradiation process (black and red lines, respectively). The angular positions, intensities, and full width at half maximum (FWHM) values of the Bragg peaks in the two curves were similar. This meant that the irradiation process did not significantly affect the internal stacks. The contaminants formed on the top surface probably can result in changes in the shape of GIXRR curve over the total reflection range. However, these changes were not significant because the size of the footprint was almost eight times that of the local area with contaminants. To identify the structural differences between the irradiation region and the area outside it, the sixth-order Bragg peak, which is observed at approximately 2.4°, was measured at different regions of the sample. The beam footprint and the size of the irradiated region were similar when the grazing angle was approximately 2.4°. The top graph in Fig. 6 shows the positions of the measured regions, which are marked with different color frames and numbers. The measured sixth-order Bragg peaks are shown in Fig. 6(b) along with the corresponding color lines and numbers. There is no observable difference in the intensities and FWHM values of the sixth-order Bragg peaks. This meant that, the structure of the multilayer sample was the same within and outside the irradiated region. That the GIXRR curves and sixth-order Bragg peaks were similar indicated the 18-h irradiation treatment did not have any observable effect on the Cr/C multilayer structure.
Fig. 6. (a) GIXRR curves for grazing angle of 0 to 3° before (black line) and after (red line) irradiation. (b) Sixth-order Bragg peaks as measured at different regions. Top graph shows locations of different measurement regions.
The soft X-ray reflectivity map was measured along the middle line shown in the top graph of Fig. 6 in order to elucidate the effects of the irradiation treatment on the optical properties. A theta-2theta scan was performed near the first Bragg peak at each point of the line. The peak reflectivity of each point composed the reflectivity map. The photon energy of the beam used for the reflectivity mapping was 1183.6 eV and the results obtained are shown in Fig. 7. A trench with a width of 9 mm was observed in the irradiated region, with the reflectivity at the center being lower than that outside irradiated region by approximately 8.6%. The reflectivity of a Cr/C multilayer having the structure parameters in Table 1 and 25-nm-thick carbonaceous contaminants on the top was calculated. It showed that the reflectivity is lower by a similar amount compared with that of a Cr/C multilayer without contaminant. This suggested that, the observed decrease in the reflectivity was attributable to the absorption and interference caused by the contaminant layer [51–53]. This degradation can be prevented by removing the carbonaceous contaminants [16,54–59]. The reason that the width of the trench was larger by 6 mm compared to the 3-mm-wide irradiated region, was that the width of the beam moved across the irradiated region was 6 mm.
Fig. 7. Reflectivity map along longer-edge direction of irradiated region measured at 1183.6 eV. Shadow region represents irradiated region.
3.3 Annealing test
The GIXRR curves of samples S1-S5 measured before the annealing treatment are shown in the top halves of Figs. 8(a)–8(e) as black solid lines. The intensities, angular positions, and FWHM values of the Bragg peaks of the five samples were similar, indicating that the periods, thickness ratios, and interfacial imperfections of the multilayers were similar. The actual Cr thickness ratios were determined by fitting the GIXRR curves using a constant-period model. The results showed that the Cr thickness ratios were close to the designed value of 0.37. The four-layer and non-periodic model was used to characterize the interfaces and determine the thicknesses of individual layers. The results are listed in Table 2. The thicknesses of the Cr-on-C and C-on-Cr interlayers were approximately 1.31 and 0.79 nm, respectively. Further, the periods were slightly lower than the designed value of 11.64 nm. The densities of Cr and C layers, determining the optical constants, were 6.67 and 2.10 g/cm3, respectively. The graphs in the lower halves of Figs. 8(a)–8(e) show the GIXRR curves measured after the annealing treatment. In principle, the phase cancellation of the X-ray reflected from each interface can suppress nth-order Bragg peak if the thickness ratio is 1/n [60]. Thus, the disappearance of the fifth-order Bragg peak in case of sample S5 after it was annealed 700 °C indicated that the Cr thickness ratio decreased approximately 0.2. In addition, the Bragg peaks of sample S5 shifted to significantly lower angles, indicating that the period had increased after annealing. To elucidate the structural changes induced at the different annealing temperatures, the GIXRR curves measured after the annealing treatment were fitted using the four-layer and non-periodic model. Based on the fitting results, shown in Table 2, it can be concluded that the density of the C layer of sample S1 decreased slightly after it was annealed at 200 °C. Previous studies have reported that when a C layer was subjected to an annealing treatment, it changed from amorphous to graphitic. This is probably the reason for the observed decrease in the density of the C layer [27,30,31]. On the other hand, the density of the Cr layer increased after annealing, probably because of the decrease in its degree of crystallization, as evidenced by the XRD results (shown later). The small changes in the densities of the C and Cr layers of Sample S1 barely affected their thicknesses. In addition, the interlayer thicknesses also remained unchanged after annealing treatment at 200 °C. When the annealing temperature was increased to 300 °C, the densities of the C and Cr layers decreased and increased further, respectively. The transition from a porous structure to one consisting of compacted arrays and vice versa can lead an increase or decrease, respectively, in the layer thickness. This is what caused the opposite changes observed in the thicknesses of the Cr and C layers. The changes in the thicknesses of the individual Cr and C layers resulted in a decrease in the Cr thickness ratio. The higher temperature caused the atoms near the interfaces to become more active, resulting in thicker interlayers. However, the change in period was only 0.04 nm because most of the expansion of the C layer was compensated for by the compression of the Cr layer. The changes in the period and interlayer thickness were positively correlated with the annealing temperature. When the annealing temperature was increased to 700 °C, the change in the period was 0.32 nm. This shifted the Bragg peaks to significantly smaller angles. The Cr thickness ratio was reduced to approximately 0.2. However, the average interface roughness remained unchanged at 0.34 nm after an annealing treatment, and it is independent of the annealing temperature. Finally, the reference sample S6 has a structure similar to those of the sample S1-S5, its fitted curve and corresponding parameters are given in Fig. 8(f) and Table 2, respectively.
Fig. 8. Measured and fitted GIXRR curves of as-deposited sample S6 (f) and samples S1-S5 annealed at different temperatures: (a) 200 (b) 300 (c) 400 (d) 500 (e) 700 °C, respectively. Scatter red lines and solid black lines represent measured and fitted GIXRR curves, respectively. Curves were fitted using four-layer and non-periodic model.
Table 2. Results of fitting of GIXRR of Cr/C multilayer samples as measured before and after annealing treatment.
The results of the small-angle X-ray scattering (SAXS) measurement for samples S1-S5 performed before and after the annealing treatment are shown in Fig. 9. The first peak is attributable to the interference caused by the specular reflection of the interfaces in the multilayer samples. The small peaks at approximately 1.68° and 2.40° are caused by the interference of the waves scattered owing to the conformal roughness of the different interfaces. The low-intensity signals (scattering wings) between the primary peaks stem from the scattering caused by the non-conformal interfacial roughness [61,62]. As shown in Fig. 9, the scattering curves for all the samples remain unchanged after the annealing treatment. This means that the interfacial roughness remained unaffected when the Cr/C multilayer samples were annealed at different temperatures, which is consistent with the fitted results.
Fig. 9. Results of small-angle X-ray scattering measurements of Cr/C multilayer samples before and after annealing treatment.
The XRD results for the as-deposited sample S6 and the annealed samples S1-S5 are shown in Fig. 10. Only one diffraction peak at approximately 44.39° was observed for all the samples. According to the corresponding powder diffraction file (PDF) of the International Center for Diffraction Data (ICDD), the diffraction peak related to the (110) crystal plane of Cr appears at 44.39°. Sample S6 shows a relatively high-intensity and narrow diffraction peak, indicating that the Cr layers in the as-deposited multilayer sample were partically crystallized. In case of sample S1, which was annealed at 200 °C, the intensity of the diffraction peak was lower, and its FWHM was higher. With an increase in the annealing temperature, the diffraction peak reduced further in the intensity and became wider. These results imply that the crystallinity and grain size of Cr layers decreased with the increasing annealing temperature. The lower crystallinity and smaller grain contributed to the formation a compact layer, leading to an increase in the layer density, as also confirmed by the GIXRR results. Another phenomenon observed was that the diffraction peaks shifted to smaller angles with the increase in the temperature. It has been reported that the interstitial atoms can increase the crystalline interplanar spacing, causing the characteristic peak to shift to smaller angles [63,64]. Here, greater number of carbon atoms diffused into the interstices of the Cr layers with the increase in the annealing temperature, resulting in an increase in the Cr (110) crystalline interplanar spacing. These results indicate that greater amount of the Cr-C mixtures was formed at the interfaces after annealing and that is the reason for the observed increase in the thickness of the interlayers.
Fig. 10. X-ray diffraction patterns of Cr/C multilayer samples before and after annealing at different temperatures.
Photon energy scans were performed at grazing angles of 3.0° and 3.6° for the as-deposited sample S6 and the annealed samples S1-S5, the measured data are shown as scatter lines in Fig. 11. The structural parameters of the reference sample S6 and sample S1-S5 before the annealing treatment were similar, with the exception of the slight differences in their layer thicknesses. Simulated results showed these slight differences only caused a difference of less than 0.4% in the reflectivities. Therefore, only sample S6 was subjected to soft X-ray reflectivity measurement as a reference to determine the reflectivities of S1-S5 before the annealing treatment. The measured curves were compared with the calculated ones (solid lines in Fig. 11) using the structure parameters obtained from GIXRR fitting. It shows that the measured and calculated curves matched very well, which means that the parameters obtained from GIXRR fittings were actual values. The red stars represent the peak points, and the corresponding coordinates including energies and reflectivities are present under the peaks. The increase in multilayer period after the annealing treatment resulted in significant differences in the positions of the peaks energy scan curves. Compared to the unannealed sample, that is S6, the annealed samples exhibited higher reflectivities. The highest peak reflectivity was observed in the case of sample S1, which was annealed at 200 °C, its peak reflectivity was 3.8% and 4.0% higher than that of S6 for grazing angles of 3.0° and 3.6°, respectively. The changes in the densities of the Cr and C layers, which enhanced the contrast in the refraction indices of the adjacent layers, can be one reason for the increasing reflectivity of S1. Moreover, with the further increases in the annealing temperature (beyond 200 °C), the Cr thickness ratio decreased. This is another reason for the reflectivity being higher after the annealing treatment [32]. However, increases in the interlayer thickness lowered the reflectivity [65,66]. This is the reason the reflectivity stopped increasing when the temperature was increased beyond 200 °C. It should be noted that the reflectivity of samples S5, which was annealed at 700 °C, was still 1.3% higher than that of the as-deposited reference sample S6 for grazing angle of 3.0°. These results confirm that the fabricated Cr/C multilayer samples had high thermal stability and that their reflectivities can be increased by subjecting them to an annealing treatment.
Fig. 11. Photon energy scans of as-deposited (S6) Cr/C multilayer and those annealed (S1-S5) at different temperatures. Grazing angles are 3.0° (a) and 3.6° (b). The scatter and solid lines are the measured and calculated data, respectively.
4. Conclusions
In this study, we investigated the stability of Cr/C multilayers when exposed to synchrotron radiation or subjected to thermal annealing. An 18-h white beam irradiation treatment with a power density of 0.1 W/mm2 resulted in the formation of local carbonaceous contaminants on the sample surface, and did not significantly affected the internal stacks. The contaminants caused in a reflectivity reduction at 1183.6 eV, which can be avoided by applying a better vacuum, or be removed by cleaning technology.
The Cr/C multilayers were subjected to annealing at 200, 300, 400, 500, 700 °C. The Cr thickness ratio decreased because of the expansion of the C layers and the compression of the Cr layers. Further, the C layers transformed from amorphous to graphitic, resulting in a porous structure. Moreover, the average interface roughness remained unchanged after the annealing treatment. However, the annealing process increased the thickness of the interlayers. The peak reflectivities at grazing angles of 3.0° and 3.6° were enhanced after the annealing treatment. This can be attributed to the larger contrast of refraction indices of the adjacent layers and not the smooth effect of C layers. The multilayer structure could withstand temperatures as high as 700 °C, and the corresponding reflectivity is higher than that of the as-deposited sample.
These results demonstrate an excellent irradiation and thermal stability of Cr/C multilayer, which can be applied on the X-ray mirrors and monochromators for the third generation synchrotron radiation source.
Funding
Shanghai Rising-Star Program (19QA1409200); National Natural Science Foundation of China (11805127, 61621001, U1732268); National Basic Research Program of China (973 Program) (2016YFA0401304).
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